1. Field of the Invention
The present invention relates to a ghost cancelling reference signal transmission/reception system for transmitting and receiving a ghost cancelling reference signal which is superposed on vertical blanking intervals of a television signal.
2. Description of Prior Arts
Conventionally, a ghost cancelling device incorporating a transversal filter is known as an effective means for cancelling ghost generated in a transmission path of television signals. This device cancels ghost by causing the transversal filter to have a filter characteristic inverse to that of the signal transmission path generating the ghost under the assumption that the signal transmission path functions as a "pseudo-filter". The device determines the filter characteristic of the signal transmission path by using a specific reference signal. Typically, a differential signal of the leading edge of a vertical synchronizing signal is used as this reference signal as shown in "Automatic Cancellation System for Multiple TV Ghosts", by S. Onishi and M. Obara, NHK Technical Report, VOL. 21, PP187-191.
Referring now to FIGS. 1(a) and 2(b), the above reference signal will be described. In FIG. 1(a), reference numeral 1 denotes the leading edge of a vertical synchronizing signal, reference numeral 2 denotes a ghost, and reference numeral 3 denotes a vertical equalizing pulse. The ghost 2 is a delayed component of the leading edge 1 of the vertical synchronizing signal. FIG. 1(b) shows signal waveforms generated by differentiating the leading edge 1 of the vertical synchronizing signal and the ghost component 2. In FIG. 1(b), reference numeral 4 denotes the differential signal of the leading edge 1 of the vertical synchronizing signal, and reference numeral 5 denotes the differential signal of the ghost component 2. These differential signals 4 and 5 indicate an impulse response of the signal transmission path. Ghost can be cancelled by using this impulse response.
In addition, NHK (Nippon Housou Kyokai or Japan Broadcast Corporation) also proposes another art of cancelling ghost using a specific reference signal called GCR (Ghost Cancel Reference), as shown in "Ghost Cancel Reference Signal" by M. Obara et al., Technical Report of the Institute of Television Engineers of Japan, RE81-6, February, 1981, pp. 33-38. The proposed art is represented in FIG. 2, in which reference numeral 6 denotes a bar signal, 7 a pulse signal, 8 a color burst signal and 9 a horizontal synchronizing signal. The pulse signal 7 is used for presuming the impulse response of the signal transmission path, and the bar signal 6 is used for detecting an occurrence of a sag generated by the transversal filter.
However, use of the vertical synchronizing signal for making up the conventional ghost cancelling reference signal still involves some problems to be solved. First, quality of synchronizing signal of the actual television broadcasting signal is not guaranteed quantitatively. The synchronizing signal part of television signal modulated into RF signal contains the largest amount of power and is vulnerable to non-linear distortion. Consequently, the relationship between waveform distortions of video and synchronizing signals due to ghost is not fixed, and therefore, it is difficult for the system using the leading edge of vertical synchronizing signal for making the ghost cancelling reference signal to totally eliminate the ghost.
Likewise, the GCR proposed by NHK, in order to securely eliminate the ghost, requires the pulse signal 7 to have a specific pulse width as small as about 1.5 T (a half-value width of 187.5 .mu.sec.) (T : sampling period). This is expressly stated in "A Study of Reference Signal for Transversal-Type Ghost Canceller" by M. Obara, Transactions of the Institute of Electronics and Communication Engineers of Japan, VOL. J68-B No. 12, December, 1985, pp. 1381-1389. Nevertheless, with the pulse signal mentioned above, information needed for compensating for close-ghost, group-delay distortion, distortion of frequency amplitude characteristic and the like that raise critical problems in the teletext system cannot easily be obtained. This is because the impulse response of the distortion in the signal transmission path appears as the collective pulse present near the original pulse, thus making it difficult for the system to separate these pulses from the original pulse.
In addition, since the pulse signal is a single pulse, its signal power is very small and thus adversely affected by a noise.
Further, in the system using the leading edge of vertical synchronizing signal as the reference signal, the system cannot correctly identify whether the delayed signal is caused by the reference signal or by another signal when the ghost delayed by more than (1/2)H (H : horizontal scanning period) exists. This will be explained with reference to FIGS. 3(a) and 3(b). FIG. 3(a) is a signal waveform showing that a ghost is delayed by more than (1/2)H. In FIG. 3(a), reference numeral 1 denotes the leading edge of a vertical synchronizing signal, 3 denotes an equalizing pulse, and 10 denotes a ghost which is a more than (1/2)H delayed equalizing pulse 3. In FIG. 3(b), reference numeral 4 denotes a differential signal of the leading edge 1 of the vertical synchronizing signal, and reference numeral 11 denotes differential signals of the ghost 10. It cannot be identified from FIGS. 3(a) and 3(b) whether the ghost 10 is derived from the equalizing pulse 3 or the leading edge 1 of the vertical synchronizing signal.
Accordingly, ghost cancellable range is (1/2)H when the leading edge of the vertical synchronizing signal is used as the reference signal. Likewise, due to the same reason as above, a ghost having a delay time more than 16.8 .mu.sec. cannot be cancelled even when the GCR proposed by NHK is used.